The new P3 props when installed properly have 'friction grip' that prevents the prop from spinning off during braking, using a metal prop hub that will not 'grip' the motor shaft could cause the prop to come off during heavy braking.

I don't think anyone's reported it happening though.
I notice that after many on and offs, they spin just as freely as the P2 ones do.
At first there seemed to be slight resistance which backed up the more grippy idea.